1. Field of the Invention
The present invention relates generally to devices or methods for filtering blood clots from blood vessels, and more particularly to vena cava blood clot filters and methods and apparatus for inserting such vena cava blood clot filters transvenously.
2. Background of the Invention
It is estimated that each year, approximately 750,000 patients in the United States suffer pulmonary embolism or passage of blood clots to the lungs. Of these, approximately 150,000 patients die each year from such pulmonary embolism. Most commonly, these clots originate in the veins of the pelvis or lower limbs. While most patients can be treated with blood thinning medications, these medications can jeopardize the wellbeing of some patients because of other, co-existing medical problems. Other patients exhibit recurrent embolism even while being treated with these medications. In these situations, a mechanical barrier is necessary to prevent such blood clots from travelling through the inferior vena cava to the heart and lungs.
Initially, surgical procedures were devised to form such a mechanical barrier. These procedures consisted of either tying a ligature around the inferior vena cava or placing a special clip around it. The surgery necessary to perform this procedure is extensive and requires a general anesthetic. Moreover, such surgical procedures significantly further jeopardize the health of an already ill patient.
Over the last fifteen years, several devices have been used to place a filtering device into the inferior vena cava using a transvenous route, commonly originating from the right jugular vein or from either femoral vein. For example, the method disclosed in U.S. Pat. No. 3,834,394 to Hunter, et al., uses a detachable balloon which is delivered to the inferior vena cava at the end of a catheter. The balloon and catheter are inserted into one of the veins in the neck using a surgical incision and passed to the lower inferior vena cava where the balloon is inflated. Once detached, the balloon occludes the inferior vena cava entirely, thereby preventing any flow of blood or blood clots to the heart. While insertion of this device avoids major abdominal surgery, it still requires a small surgical procedure to be performed in order to expose a neck vein. The balloon occludes the inferior vena cava completely, resulting in swelling of the lower extremities until collateral circulation develops around the balloon. With time, these collateral channels may become large enough to permit life threatening emboli to pass to the lung.
Another device for preventing pulmonary embolism but which does not require total occlusion of the inferior vena cava is an implantable cone-shaped filter device consisting of six spokes with sharpened points at the end and connected together at the other end by a central hub. A thin membrane with 4 mm. holes covers the device. The umbrella-like device is folded into a cylindrical capsule connected to the end of a catheter. This device is described in U.S. Pat. No. 3,540,431, to Mobin-Uddin. This device also requires a surgical cutdown on a major right neck vein for access to the venous system. The device and delivery capsule are positioned in the inferior vena cava and released by pushing the device out of the capsule. While the device acts as an efficient filter, approximately 60% of the patients using the Mobin-Uddin filter develop occlusion of the inferior vena cava, sometimes resulting in severe swelling of the legs. Furthermore, instances of migration of the filter to the heart have been reported; such instances present a high mortality risk.
The Hunter balloon and the Mobin-Uddin umbrella suffer from similar disadvantages in that they require a surgical procedure on the neck for exposure of a vein into which the filter may be passed. Furthermore, morbidity from occlusion of the inferior vena cava could be severe. A device which could be easily inserted from the femoral approach using standard angiographic techniques, and thereby avoid surgery, would be desirable. Ideally, the device should not totally occlude the inferior vena cava or be thrombogenic. It should also be securely anchored within the inferior vena cava to prevent migration.
U.S. Pat. No. 3,952,747, to Kimmel, discloses a blood vessel filter and filter insertion instrument which overcome some of the disadvantages of the previous two devices. The Kimmel patent describes a device which may be inserted either from the jugular or femoral approach using a surgical exposure of a major vein. The conical shaped device consists of six strands of wire each connected to a hub at one end and having recurved hooks on the other end. The device is loaded into a cylindrical delivery capsule which is connected to a catheter. The delivery capsule measures 6 mm. in diameter and 5 cm. in length. Because of its size, a surgical exposure of the vein is necessary for introduction of the delivery capsule into the vascular system. More recently, the delivery capsule has been introduced into the vascular system through a large catheter using angiographic techniques. However, this technique has been shown to significantly injure the vein at the introduction site. Sometimes it may not be possible to pass the capsule from below through tortuous pelvic veins into the inferior vena cava because of the inflexibility of the capsule. The filter engages the wall of the vein at one end and therefore often tilts to one side. It is very difficult to deliver the filter in a manner that maintains the longitudinal axis of the filter centered along the longitudinal axis of the vena cava. A tilted filter has been shown to be less efficient at capturing blood clots. Migration of the filter has not been a problem.
Another method of preventing pulmonary emboli from reaching the lungs is a device disclosed in U.S. Pat. No. 4,425,908, to Simon. This device uses the thermal shape memory properties of Nitinol to deploy the filter following delivery. The filter consists of seven wires banded at one end and also in the middle. The wires between these two points form a predetermined filter mesh derived from the thermal memory. The free-ends of the wires form anchoring points which radially engage the inferior vena cava. The device may be inserted through a jugular or femoral vein approach using standard angiographic catheters. The device relies on the thermal shape memory properties of the Nitinol wire to form an effective filter following delivery. It is not yet clear whether the filter disclosed in the Simon patent will be biocompatible in humans or if it will be thrombogenic. Concerns exist regarding its reliability when stored at different temperatures and also whether the material can be manufactured with the same consistency.
U.S. Pat. No. 4,494,531, to Gianturco, also discloses a blood vessel filter which can be inserted through angiographic catheters. The device consists of a number of strands of wire which are interconnected and wadded together to form a curly wire mesh. The filter includes a number of projections which serve as an anchoring means for anchoring the filter at a suitable body location within the inferior vena cava. Problems with the device include migration and demonstration invitro of filtering inefficiency. The random nature of the filtering mesh makes it difficult to assess the overall efficacy. Perforation of the anchoring limbs through the vena cava has also been described.
A device described by Gunther et al. in a 1985 technical article consists of a helical basket made of a number of wires and radially placed legs. Originally, it was intended to be implanted temporarily in the inferior vena cava until the patient's risk of pulmonary embolism had passed. Limited clinical experience is available.
The blood clot filter device and related delivery apparatus disclosed in the present invention, overcome the disadvantages associated with the prior art by employing a nonocclusive filter which is designed to be inserted into the vena cava using normal percutaneous catheterization techniques through a femoral or jugular approach. Thus the need for surgery is totally eliminated. The device is self-centering and has a positive mechanical locking system. This system does not require the patient to be at a given temperature in order for the filter to form its shape. Moreover, it is made of metals which have been shown to be biocompatible when used in other devices such as pacemakers and inferior vena cava filters. This is not true of the filter disclosed by Simon. The filter configuration is predetermined and not random as described by Gianturco.
Accordingly,it is an object of the present invention to provide a blood clot filter which may be implanted using normal percutaneous angiographic catheter techniques through either a femoral or jugular approach.
It is a further object of the present invention to provide a blood clot filter which is designed to be placed within the inferior vena cava below the renal veins.
It is yet a further object of the present invention to provide a blood clot filter which does not obstruct blood flow within the blood vessel at any time.
It is still a further object of the present invention to provide a blood clot filter which will not cause thrombus formation or emboli after implantation.
An additional object of the present invention is to provide a blood clot filter which is capable of being securely anchored within the blood vessel.
It is a further object of the present invention to provide a blood clot filter which forms its shape using mechanically induced conversion of straight wires into a filter mesh which may accommodate vena cavas of varying sizes.
It is another object of the present invention to provide such a blood clot filter which uses well-known biocompatible materials and which avoids reliance upon thermal memory shape characteristics, thereby providing a reliable and less expensive filter.